1. Field of the Invention
The present invention relates to a method for voltage limitation and to a circuit for voltage limitation in a transponder.
2. Description of the Background Art
Methods and circuits are known to one skilled in RFID (RFID=Radio frequency Identification). RFID is, for example, a transmitting resonant circuit that may supply an inductively coupled receiving resonant circuit with power and reads data via the receiving resonant circuit. Such connections are employed, for instance, for object identification, whereby a transmitting resonant circuit of a reading device (reader) responds to an object marked with a label (tag) via a receiving resonant circuit and retrieves information.
To establish contact, the transmitting resonant circuit of the reading device generates a high-frequency magnetic field, which induces an alternating voltage in an inductor of a receiving resonant circuit located near the reading device. The alternating voltage induced in the receiving resonant circuit is rectified and is used, for instance, to supply power to an integrated circuit connected to the receiving resonant circuit. In addition, a clock frequency, which can be made available to the integrated circuit, for example, to a microprocessor and/or a memory element as the system clock, can also be derived from the induced alternating voltage. By supplementing the inductor of the transmitting resonant circuit and/or receiving resonant circuit with capacitors, particularly with parallel capacitors, for resonant circuits, resonance effects are achieved which considerably improve the efficiency of the power transmission.
A transmission of data from the reading device to the receiving resonant circuit (downlink) can occur, for instance, by turning the magnetic field on and off. For data transfer in the opposite direction from the receiving resonant circuit to the reading device, a load modulation can be used, which requires a sufficient proximity (distance less than 0.16*wavelength) of the transmitting and receiving resonant circuit. With sufficient proximity, transformer coupling occurs in which the power uptake of the receiving coil is revealed by a feedback to the transmitting resonant circuit in voltage changes at the transmitting resonant circuit. Controlled modulations of the load, thus, the impedance of the receiving resonant circuit, cause voltage changes in the transmitting resonant circuit, which can be evaluated for a data transmission.
As the quality of the inductors used in the receiving resonant circuit improves, when the ratio of reactance to effective resistance increases, the attenuation of the resonant circuit and the width of the resonance curve decline. The use of higher quality coils therefore causes greater frequency selectivity and, at the same voltage on the reader side, a higher voltage on the tag side, which broadens the range of the communication connection. At small distances between the reader and tag, depending on the transmitting power, such high voltages can be induced in the tag that an integrated circuit present in the tag can be destroyed.
German Patent Application DE 10 2004 020 816 A1, which corresponds to U.S. Publication No. 20050237123, which is incorporated herein by reference, discloses a reduction or limitation of the voltage at the receiving resonant circuit to specific values, which is described hereafter as the first clamping voltage. Furthermore, within the framework of the load modulation, switching is undertaken between a first voltage, i.e., the first clamping voltage, and a second lower voltage. For this purpose, depletion layer elements are inserted between the resonant circuit terminals and a reference or ground potential. For example, a lower clamping voltage is realized when the forward voltage drops across the depletion layer elements, whereby in a first approximation, the voltage drop is independent of the current due to the exponential dependence of the current on the voltage.
As a result, the depletion layer elements act as a reliable limitation of the resonant circuit voltage to a corresponding value, even at high coil currents. This is of particular importance in systems with high-quality inductors, which at close spatial proximity of the transmitting resonant circuit and the receiving resonant circuit could otherwise cause undesirably high voltages.
The upper clamping voltage can be realized by a Zener diode, connected in series, with a reversed forward direction that can be short-circuited in a controlled or connected manner. In the short-circuit mode, the described limitation occurs to the lower clamping voltage, whereas in a non-short-circuit mode, the breakdown voltage of the Zener diode provides an additional voltage offset, which in sum with the aforementioned forward voltages defines an upper clamping voltage. In a mode when the Zener diode is short-circuited, a comparatively large current emerges from the receiving resonant circuit, which corresponds to the loaded mode of the resonant circuit. The current drain from the resonant circuit as well as the demand on the resonant circuit by opening the short circuit via the Zener diode is decreased accordingly.
A problem has been observed with the load modulation from conventional circuits, which is, when during the activation of modulation, that is, when the resonant circuit voltage is limited to the lower clamping voltage, a high coil current is induced, the current drains typically through a bridging of Zener diodes and the remaining depletion layer elements that are switched in a forward direction, whereby the resonant circuit voltage can fall below the lower clamping voltage and even below a threshold value that is used for detecting oscillations (pulses) of the resonant circuit voltage. With unfavorable phase conditions, therefore, it can happen that when the load is turned on, the voltage of the transmitting resonant circuit drops below a detection threshold for one or several periods due to feedback, which corrupts the information transmission. This can cause data loss during information transmission to the reading device.
If, during a high-induced coil current, the modulation is turned on, the depletion layer elements assure a limitation of the resonant current voltage to a value predetermined by the depletion layer elements. In this phase, the diodes function like a direct-current source and thus do not attenuate the coil current sufficiently so that the induced oscillation is altered. This results in a widening of the directly adjacent clock pulse phase (pulse widening), which leads at least to a partial cancellation of the sequential oscillation. This is manifested in that at least one oscillation in the amplitude is too small for a preset detection threshold.